Low voltage instrument preamplifier, based on ECH83 tube.
I've got several good-looking ECH83
tubes valves and as I've had lots of fun experimenting with American low voltage space charge tubes (see Space charger amplifier), I wanted to do something with European valves too.
The preamp sounds very nice even under low voltages, not a hi gain distortion but a nice warm tone with plenty of output volume. You can even build it into a stompbox and put it on your pedal board with 9V power supply! The ECH83 is only getting a little warm, so the cooling is not issue here, it may be hidden inside a stompbox. And I've seen Telefunken ECH83's go for around $2 each on ebay, so you can impress everybody with the diamond logo.
Both triode and Heptode sections are intended to work in grid bias mode, with grid resistor of 47K for triode section and 1M for heptode. These values do not change through the datasheet, so we can only play with load resistors. Here's the schematics I came up with:
The power supply is basically a 9V adapter - something like a guitar pedal power supply, capable of around 500mA. If you have a higher voltage, like +20V, you may want to change the plate and grid load resistors values. The tested working values for 20V are:
100K triode plate load resistor (pin 8)
100K heptode plate load resistor (pin 6)
I've drawn it here in two basic variations, with one being a simple sort of overdrive effect, and the other as a full preamp with less gain (due to the gain loss in the tone stack). The most "innovative" part here is the gain control. I've tried all sorts of traditional gain controls, but they're not working good with heptodes, having almost no effect on sound. So in my design the heptode gain control changes the voltage of the control grids. I'm not sure you can simply wire high voltage rail to a potentiometer in conventional +250V and higher designs, so here's the advantage of low voltage tubes. Sorry, valves.
The gain control works pretty good and it provides very dramatic changes. At maximum, when the control grid voltages are the highest, the sound is very rich and full, with some overdrive. Of course, if you have the tone stack, you get only half the overdrive and a more chimey sound. As you rotate the gain towards the minimum, the control plate voltage goes down to about 1V at the end of the run. The result is gain reduction at first, and then it enters "choking" zone. The tube hardly transmits the signal, it gets distorted, grainy and fuzzy. It sounds different from tube saturation, I guess Eric Barbour might like it.
There was another attempt on ECH83 preamp proposed at diyaudio.com forum and one guy (user Cursor) built a hybrid guitar amplifier with TDA2030A power IC output and the ECH83 preamp under 20V rail. He built the preamp after I've posted an untested scheme at the forum. I'm not posting his scheme because it had several mistakes. He also reported his gain control wasn't working, which I discovered as well later. My schematics are tested and working.
If you really want to print it on paper, here is the PDF file with both versions:
ECH83 is a 9-pin triode-heptode, with 6.3V heater, originally designed for car radios. While american car radio tubes are all 12V, these European beauties are able to live with 6.3V anode voltages.
ECH83 datasheet - here you can see the datasheet of the valve.
The common use for heptode is frequency changer, but as soon as the datasheet suggested values for RF/IF amplification I've decided to give it a try as AF amplifier. Well, it worked.
I've used a very nice toy automobile for the enclosure. The car is made of some sort of metal, and I've drilled holes for jacks and pots. The valve sits inside the cabine, but it's a bit too tall so it sticks above. The most important part, however, are the lights. It's got a stompbox 3PDT bypass switch, so when it's bypassed, the rear red lights are on, and when the preamp is active, the front lights are on. Now that's what I call a real valve preamp!